A photodissociated region associated with the compact Hıı region near GGD 12-15

Abstract

We present Very Large Array (VLA) continuum (2, 3.6, and 20 cm) and line (H I 21 cm, C92a, and H92a) observations toward the obscured cometary-like H II region located near the optical nebulosities GGD 12È15. We Ðnd that the H92a recombination line proÐle is asymmetric, probably due to a superposition of line emission from the H II region and from an underlying partially ionized medium (H0). The observed kinematics of the ionized gas suggests that the H II region is undergoing a champagne Ñow. The C92a observations show that the C` emission arises from an extended region of D20A in size that is closely associated with the H II region. The C` emission has a line center velocity of 11.9 km s~1, similar to the velocity of the ambient molecular cloud (D11 km s~1). The 21 cm H I spectrum shows emission and absorption line components. The H I line in emission is detected from a region of D23A in diameter, with a center velocity of 15.4 km s~1 and peaks to the east of the H II region. The absorption feature is unresolved (¹22A), has a line center velocity of 11.7 km s~1, and lies in front of the H II region. Both the C92a and H I emissions are interpreted as arising from a photodissociated region (PDR) around the cometary H II region. An isothermal model has been used to derive the physical parameters of the photodissociated hydrogen gas around the H II region. We derive that the PDR region has an excitation temperature of D330 K, a hydrogen column density of D6]1021 cm~2, a H I number density of D1.5]104 cm~3, and a H I mass of D5 M The mass in photodissociated hydro- _. gen is about 3 orders of magnitude larger than the mass in ionized hydrogen (D2]10~3 M_). Subject headings : H II regions È ISM: abundances È ISM: individual (GGD 12È15) È